This invention relates generally to remote object position and orientation determining systems employing electromagnetic coupling and, more particularly, to a unique sensing and processing technique for such systems.
Remote object position and orientation determining systems employing AC electromagnetic coupling are disclosed in U.S. Pat. No. 3,868,565 issued to Jack Kuipers for an OBJECT TRACKING AND ORIENTATION DETERMINING MEANS, SYSTEM AND PROCESS and U.S. Pat. No. 4,054,881 issued to Frederick Raab for a REMOTE OBJECT POSITION LOCATOR. Such systems traditionally have a source assembly that includes a plurality, typically three, of concentrically positioned, orthogonal field generating antennas for generating a plurality of electromagnetic fields. Signals are applied to the field generating antennas that are multiplexed so that the resulting electromagnetic fields are distinguishable from one another. Located at the remote object is a sensor having a plurality, also typically three, of concentrically positioned, orthogonal receiving antennas for receiving the electromagnetic fields generated by the transmitting antenna and producing signals corresponding to the received electromagnetic fields. A processor resolves the signals produced by the receiving antenna into remote object position and orientation in the reference coordinate frame of the source.
In U.S. Pat. No. 4,945,305 issued to Ernest B. Blood for a DEVICE FOR QUANTITATIVELY MEASURING THE RELATIVE POSITION AND ORIENTATION OF TWO BODIES IN THE PRESENCE OF METALS UTILIZING DIRECT CURRENT MAGNETIC FIELDS, a remote object position and orientation determining system is disclosed in which the transmitting antennas are driven sequentially by a pulsed, direct current signal. The generated electromagnetic fields are sensed by a DC-field-sensitive sensor in each of the three orthogonal components of the sensor reference coordinate frame and are resolved into remote object position and orientation relative to the source reference coordinate frame. The purpose of the system disclosed in Blood is to reduce field distortions resulting from the decay of eddy currents induced in electrically conductive materials by magnetic fields. Blood teaches that these disadvantages are overcome by applying a pulsed-DC signal to a source axis, which will induce an eddy current in any surrounding conducting metal only at the beginning of the pulse. The Blood system waits enough time for the eddy current to decay before measuring the transmitted flux. Alternatively, the received signal is measured several times as the eddy current is dying out and curve-fitted to an exponential decay math function in order to remove the effect of the eddy current field distortion.
The Blood system is not without its drawbacks. In order to measure a DC field, a complex, bulky and expensive active sensor must be employed. An example of a sensor that is sufficiently sensitive for most applications is a flux-gate active sensor. However, the complexity, and, hence, the bulk of such active sensors do not adapt well to many applications, such as to a digitizer of the type disclosed in U.S. Pat. No. 4,945,305. The sensor is positioned on a stylus that is held in the user's hand. An active flux-gate sensor is not only more bulky but requires an increase in the number of signals exchanged, and hence wires, between the sensor and the electronics unit. Another significant limitation of the system disclosed in Blood is the poor signal-to-noise performance characteristics of active DC sensors. The poor signal-to-noise ratio of active DC sensors as compared with passive AC sensors limits the range at which the system disclosed in Blood may operate with satisfactory accuracy.
Because the sensor employed in Blood is a DC field sensor, the sensor measures a composite static magnetic field made up of the pulsed-DC field from the source and a constant DC field surrounding the earth, i.e., the earth's magnetic field. Prior to resolving the sensor measurements into position and orientation, the Blood system must subtract out the effect of the earth's magnetic field. Because, at nominal source/sensor spacing, the earth's field is an order of magnitude greater than the DC field generated by the source, it must resort to hardware to subtract out the earth's field. This is necessary in order to avoid limitations in the dynamic range of the analog-to-digital converter. Furthermore, a quiescent period during which no source coil is being excited is necessary in order to allow the magnitude of the earth's magnetic field to be measured. All of this adds further complexity and error to the Blood system.